The present invention relates to a detector for accurately detecting changes in the level of an amplifier which amplifies an input signal whose level changes and, more particularly, to a detector for an amplifier included in a mobile communication system.
A prerequisite with a mobile communication system is that changes in the output level of an amplifier be accurately detected to insure accurate control over the output of the amplifier. To meet this requirement, it has been customary to use a coupling circuit for extracting the output of the amplifier, to which an input signal whose level changes is applied, in a predetermined coupling amount. A detector detects the output of the coupling circuit to thereby detect changes in the output level of the amplifier. The detector, however, cannot detect the output of the amplifier accurately when the output level of the amplifier noticeably changes. This problem is particularly serious in the case of a mobile communication system. Specifically, in a mobile communication system, since a mobile station freely moves throughout a cell, the electric field of a signal received by a base station changes with the varying position of the mobile station. For example, when the mobile station is located near the base station, both the input to the base station and the input to the mobile station are excessive. This would generate undesirable waves due to cross modulation or similar cause. In the light of such a situation, it is a common practice with a mobile communication system to change the output level of the amplifier over a substantial range, e.g., even by 30 dB, so that the electric field of an input signal may remain constant. It is, therefore, necessary for the detector to detect changes in the output level of the amplifier with accuracy. However, a conventional detector using a diode cannot detect the changes accurately since the diode characteristic is not linear over a substantial range.